The present invention relates generally to a fuel cell system having a combustor for heating a fuel reformer. In particular, the present invention relates to a catalytic combustor having a flame combustor for pre-heating the catalyst during start-up of the fuel cell system.
H2xe2x80x94O2 (air) fuel cells are well known in the art and have been proposed as a power source for many applications. There are several different types of H2xe2x80x94O2 fuel cells including acid-type, alkaline-type, molten-carbonate-type and solid-oxide type. So-called PEM (proton exchange membrane) fuel cells [a.k.a. SPE (solid polymer electrolyte) fuel cells] are of the acid-type, potentially have high power and low weight, and accordingly are desirable for mobile applications (e.g., electric vehicles). PEM fuel cells are well known in the art, and include a xe2x80x9cmembrane electrode assemblyxe2x80x9d (a.k.a. MEA) comprising a thin, proton transmissive, solid polymer membrane-electrolyte having an anode on one of its faces and a cathode on the opposite face. In PEM fuel cells hydrogen is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be in the form of pure O2 or air (O2/N2).
For vehicular applications, it is desirable to use a liquid fuel such as a low molecular weight alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the fuel for the vehicle owing to the case of onboard storage of liquid fuels and the existence of a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished heterogeneously within a chemical fuel processor, known as a reformer, that provides thermal energy throughout a catalyst mass and yields a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide according to the reaction:
CH3OH+H2Oxe2x86x92CO2+3H2
The reforming reaction is an endothermic reaction that requires external heat for the reaction to occur. The heat required to produce enough hydrogen varies with the demand put on the fuel cell system at any given point in time. Accordingly, the heating means for the reformer must be capable of operating over a wide range of heat outputs. Heating the reformers with heat generated externally from either a flame combustor or a catalytic combustor is known. The present invention relates to an improved catalytic combustor, and the integration thereof with a fuel cell system so as to fuel the combustor with unreformed liquid fuel, hydrogen-containing anode exhaust gas, or both at different times in its operating cycle depending on the electrical demand placed on the system.
The acceptance of fuel cells by vehicle owners will be governed, in part, by their experience with vehicles powered by an internal combustion engine. Consumers have grown accustomed to the relatively quick starting times of engines. Thus, one challenge facing fuel cell designers is to provide a similar relatively quick start up time for fuel cells. This is made difficult by the relatively high operating temperature of some of the components of fuel cells such as the fuel reformer and the primary reactor of the fuel processor.
In order to reduce the start-up time required to heat the catalyst to its light-off temperature (between 150xc2x0 C. to 250xc2x0 C.), it is known to equip the catalytic combustor with an electrically-powered heating element. Unfortunately, such electrically heated catalyst systems require a relatively large electric power input (typically 2-4 kilowatts at 12-24 volts and 160-240 amps) at start-up and can potentially damage the catalyst bed (due to thermal shock) if not properly controlled. Thus, the expense associated with the system componentry (i.e., associated batteries and high current power switching elements) and the potential for reduced catalyst durability due to repetitive start-up requirements severely limit the use of electrical pre-heat systems in association with most catalytic combustors.
Accordingly, a need exists in the fuel cell industry to develop low-cost and low-power pre-heat systems for use with catalyst-type combustors.
The present invention provides a combustor for a fuel processor which integrates a burner and a catalyst. The burner is utilized to quickly heat the catalyst to a light-off temperature to prepare it for normal operation. The heated catalyst is then used to react anode exhaust with air or cathode exhaust under normal operation.